An ordinary boiler arrangement of the prior art is comprised of a furnace to which fuel, bed material, and combustion air are introduced. When combusting the fuel, heat is generated and both bottom ash and flue gases are formed. The flue gases are taken to a separator that separates solid particles from the gases, and with the solid particles being returned back to the furnace. Both the furnace and the separator are provided with heat exchange surfaces comprising water or steam tubes to collect heat from the flue gases and the solids moving in the boiler. The flue gases are taken from the separator to further heat recovery devices, such as superheaters or reheaters, where the heat still available in the flue gases is used to further heat the steam. The heat is transferred into use, such as, for instance, for generating electricity by means of steam turbines and generators, in the form of high temperature steam. After the superheaters and reheaters, the flue gases flow through an economizer that, again, collects heat from the flue gases to boiler feed water, i.e., water or condensate that is returning to the boiler from the use, for instance, from the turbines. The most often final step of collecting heat from the flue gases takes place in the combustion air preheater, where the flue gas heat is used to heat the air that is used as combustion air in the furnace. The preheater is normally a rotary or tubular preheater. The combustion air preheater is followed in the flue gas path by an electrostatic filter/precipitator that separates any solid particles left in the flue gases before the flue gases are vented to the atmosphere by means of a flue gas fan via a stack.
The boiler feed water entering the economizer originates, typically, as already mentioned above, from the use in steam turbines and a condensor downstream of the steam turbines. The condensate is first heated by steam extracted from the steam turbines by means of one or more low-pressure preheaters until the condensate is introduced into the feed water tank, which is used to deaerate the water, and sometimes, to heat the water further, before pumping it towards the economizer. The feed water pumped from the feed water tank by means of a pump may further be heated by means of a high pressure preheater before entering the economizer.
The bottom of the furnace is provided with a grid for introducing combustion or suspending or fluidizing gas, called primary or combustion air, into the furnace, and for removing ash and other debris from the furnace. The common name for the material discharged from the boiler furnace through the grid is bottom ash. It contains non-burning material, clinker, unburnt fuel particles, etc. Normally, the bottom ash is discharged in a water filled trough or to water- or air-cooled conveyors where it gets cooled. The cooled bottom ash is then taken out of the plant to be dumped, or sometimes used as construction material.
Thus, in conventional prior art boilers, the loss of heat energy in the discharge of the bottom ash forms a significant portion of boiler losses. This is even more so with certain high ash content fuels, i.e., when the estimated bottom ash content of the fuel is high or when there is a need to remove or to circulate coarse or otherwise inappropriate bed material from the furnace. The reason for the high loss of energy is that the bottom ash to be removed from the furnace is high in temperature, usually, about 700 to about 800° C. For example, if the bottom ash flow from the boiler is 10 kg/s at a temperature of 725° C., using the reference temperature of 25° C. and the heat capacity for ash of 1 kJ/kg, an energy loss of 7 MW while discharging bottom ash can be expected.
The prior art shows, however, a couple of systems in which some heat of the bottom ash is recovered. Published European Patent Application No. 0 471 055 B1 discloses a boiler arrangement in which the bottom ash is discharged from a grid area onto a moving, specifically-designed steel belt. The heat recovery is arranged such that cooling air is made to flow concurrent to the bottom ash flow and to the steel belt movement, so that the heated air finally enters the furnace.
Published International Patent Application No. WO 2007/134874 A1 discusses a boiler arrangement in which the steel belt bottom ash discharge of the above-referenced document is still used, as well as the countercurrent air flow. This publication, however, further teaches that, for cooling the bottom ash, water is sprayed on the bottom ash, so that the generated steam finally enters the furnace together with the heated air flow.
The heat recovery technology discussed in the above-mentioned patent documents, though teaching the possibility of recovering heat from the bottom ash discharge, is not that efficient. Especially, when a larger amount of bottom ash is discharged, it is clear that the countercurrent air flow is not able to cool the ash sufficiently. And, even if additional water spraying is used, there are two risks. The first is that the ash will not be cooled sufficiently, and secondly, if the ash is cooled sufficiently, there may be too much steam to be introduced into the furnace. Additionally, mostly due to the inefficiency of air acting as the heat transfer medium, the size of the devices functioning even in a satisfactory manner are very large, and expensive. And, further, to be able to cool the bottom ash sufficiently, a large amount of air is needed, the use of which as combustion air may interfere negatively with the combustion air system of the boiler.